Distribution of boron, lithium and beryllium in ocean island basalts from French Polynesia: implications for the B/Be and Li/Be ratios as tracers of subducted components

Abstract

The study focuses on the distribution of B, Be, Li, rare earth elements (REE), high-field-strength elements (HFSE), Th, U and Pb in fresh and hydrothermally altered ocean island basalts (OIB) from French Polynesia, and evaluates B/Be and Li/Be ratios as potential tracers of subducted components in the mantle. Hydrothermal solutions affecting the rocks during cooling were derived from meteoric water, sea water and magmatic fluids. The concentrations of REE, HFSE, Th and Be in the OIB were not affected by secondary processes except during advanced stages of subaerial hydrothermal alteration where saponite was a dominant secondary phase. This alteration modified the contents of these elements, changed REE patterns and produced a positive Ce anomaly. The subaerial and submarine hydrothermal alteration (T ~ 70-100~ may change U concentrations in OIB, whereas Pb was only marginally redistributed during alteration.Boron was enriched during submarine and subaerial hydrothermal alteration but was not noticeably affected in basalts altered by magmatic fluids at T > 200~ Like B, the mobility of Li during the alteration varies with fluid temperature. Lithium became enriched in the basalts during advanced stages of lower T hydrothermal alteration (<100~ However, this element was partly removed from the rocks during higher T alteration (>200~ by magmatic fluids. Boron, Be and Li behave as incompatible trace elements in basaltic magmas. Beryllium content in primitive mantle is estimated to be 0.07 ppm. Fresh Polynesian OIB have low abundances of B and Li and low B/Be (2-5) and Li/Be (2.5-5) ratios compared with volcanic arc rocks, marine sediments and altered oceanic crust. Various OIB including even those which have HIMU-and EMaffinities have similar overlapping B/Be and Li/Be ratios. Both B and Li are probably stripped from a lithospheric slab during subduction-related metamorphism and are, thus, not involved in deep mantle recycling. The mantle-normalized trace element abundances of MORB and OIB usually display patterns characterized by negative B, Pb and Li anomalies. The patterns of continental crust and crustal rocks have distinct positive anomalies for these elements whereas continental basaltic rocks have variable relative abundances of B, Pb and Li. The anomalies of these elements in basalts can be useful in discriminating their tectonic setting and constraining the mantle source regions of basalts.

Li, Be, and B abundances in minerals of peridotite xenoliths from Marsabit (Kenya): Disequilibrium processes and implications for subduction zone signatures

“…Such a calculated melt in equilibrium with the completely homogenized Group II xenolith Ke 1958/20 would have 4.9 μ g/g Li, 3.2 μ g/g Be and 5.8 μ g/g B (using the average Li, Be and B of ol‐I, opx‐I, and cpx‐I, bulk partition coefficients based on mineral/melt D values of Brenan et al [1998a] and relative modal abundances as given in Table 1; spinel was neglected due to low modal abundances). These values compare well with typical light element budgets of OIB type intraplate basaltic rocks [e.g., Ryan and Langmuir , 1987; Dostal et al , 1996; Ryan et al , 1996; Ryan and Kyle , 2004; Tanaka and Nakamura , 2005; Gméling et al , 2007]. This also holds true for Ce/B and B/Be of the hypothetical melt (Figure 9b), but not for the Li/Yb ratio, which is significantly lower (∼0.5) than in any typical mantle‐derived melt (Figure 9a).…”

Incompatible element ratios in French Polynesia basalts: describing mantle component fingerprints

“…For consistency, samples were selected from suites analysed mainly at the Centre Geologique et Geophysique, Universite de Montpellier II and St Mary's University, Halifax (Dostal et al 1983(Dostal et al , 1995(Dostal et al , 1998Liotard et al 1986;Dupuy et al 1987Dupuy et al , 1988Dupuy et al , 1989Dupuy et al , 1992Dupuy et al , 1993. As a result, chemical differences between samples are likely to reflect differences in composition as opposed to being artifacts of different analytical methods.…”

Geochemical heterogeneity and element mobility in deeply subducted oceanic crust; insights from high-pressure mafic rocks from New Caledonia

“…Be, B, Li, As, and Sb systematics of high-P, low-T mafic rocks of the Pouébo Terrane. Data for the MORB and OIB fields is from Ludden and Thompson (1979), Chan et al (1992), Dostal et al (1996), Leeman (1996), Leeman and Sisson (1996), Noll et al (1996), Teagle et al (1996), Kamenetsky et al (2000), Neal et al (2002) and Ryan (2002). alent rocks is required to qualitatively evaluate element mobility during subduction.…”